Magnetometer Viby
Viby (Sollentuna), Sweden · geographic coordinates: 59°27' N 17°54 E · geomagnetic coordinates (2017): 57.69°N, 106.22°E

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The device

SAM Viby facilitates Speake & Co Llanfapley FGM-3 fluxgate sensors for measuring the local magnetic field. These sensors were produced by the late Bill Speake until early 2017. The magnetometer is located in Viby (Sollentuna), about 15 km North of Stockholm.

Geographic coordinates: 59o27min N 17o54min E
Geomagnetic coordinates (2017): 57.69oN, 106.22oE

Magnetic declination: 5 57.60' East (IGRF-12 Model , 2015)
Annual Change (minutes/year): 10.3 '/y East

Construction and installation of the instrument

Due to a required maintenance of SAM Haimhausen, a magnetometer which has been run by me 20 km north of Munich, Germany, my interest in magnetometers got triggered again. So I decided to install another magnetometer close to my residence north of Stockholm and obviously closer to the auroral zone. Just 40 kilometers south of the 60th parallel, this location with about 15 northern-light appearances every season seems well suited for geomagnetic measurements in order to receive timely alerts for observing the aurora.

Ralf Pitscheneder generously donated his mothballed 2-axis SAM magnetometer for further use as SAM Viby. Ralf's magnetometer had been operated from 2004-2010 in Munich as well. We have brought it to Sweden and installed it during the Christmas holidays 2016. The device now takes measurements in a stable configuration.

The two FGM-3 magnetic field sensors were reused and initially installed in a 40-liter cardboard box in my storage, which featured reasonable temperature stability (the magnetic field sensors are quite sensitive to temperature changes on the 0.1-degree Centigrade level). The location is as magnetically quiet as a suburban neighborhood can be, but SAM Viby is employing powerful software tools to counteract effects of man-made disturbances on the measurements. Recently, the observatory was fitted with a temperature sensor, and the temperature insulation of the observatory was improved by filling the box with bean-bag styrofoam spheres (which are produced by a local chap). Recently (March 2017), the insulation has been further improved by using EPS/XPS styrofoam detector holders and embedding the setup in an additional EPS box: The magnetometer is now installed inside a PORPAC 1182400 styrofoam box (20g/l density, with a wall thickness of 5cm and a thermal conductivity of 0.035 W/(m·K) 100.80 liters outside volume, 47.25 liters inside volume). This box is clad with two layers of 4-mm Wiedland ThermoReflekt Super Polynum (equivalent thermal resistance of four additional layers of styrofoam). The fluxgate sensors are housed by 4.5✕4✕12 cm Bachl XPS boxes and installed inside the box on a wooden support. The support is immersed in a bath of 2-4 mm diameter EPS spheres. The total thermal resistance of the box amounts to RT = 7.0 m2·K/W, equal to 25cm of styrofoam. The EPS sphere bath and the inner XPS shielding amount to an additional RT of about 2 m2·K/W. Along with this improved housing, a third FGM-3 magnetic flux sensor for the vertical component of the magnetic field has been installed, as well as a second temperature sensor.

Along the relocation into the styrofoam box, the observatory has been upgraded with a third sensor and now reads three components of the Earth's magnetic field along a geographical coordinate system (X/Y/Z) The accuracy of the relative measurements is given by ±1-2 nT (sensor accuracy), and probably ≈5 nT man-made disturbances. Relative measurements are relevant for producing alerts for northern lights observations. SAM Viby also does absolute magnetic field measurements. These are currently calibrated using the WMM2015 model which is evaluated on daily basis. The model bears absolute uncertainties of ≈135 nT in each coordinate.

Since summer 2018, the system runs on a UPS (uninterruptable power supply), so that data are obtained independently of power cuts.

Some pictures of the magnetometer. More thorough documentation to come soon.

Some diagnostic plots

A first trial to assess the temperature dependence of the measurements: This plot shows the magnetic flux vs. the ambient outdoor temperature. Dedicated temperature sensors directly at the fluxgate sensors will be installed soon.

Darrel Emerson did exhaustive tests on the charateristics, thermal dependencies and thermal and long-term effects. For determining the temperature dependence of the magnetic field measurements, he used a 50-day dataset, with 24-hr binning and a dynamical range of 5 deg C, concluding a dependency of -46 nT/deg F = -82.8 nT/deg C. My very naive first approach, based on a minute-binned correlation within 96 hours and a temperature range of 2 deg C, results in a dependency of -160 nT/deg C. The difference in coefficients may be due to the exact location where the temperature is measured. I will continue investigating this as I continue improving the insulation of the magnetometer.

Another interesting plot contains the raw measurements without compensating for offsets, as caused by passing cars, slamming fridge doors, operating the washing machine, getting skies out of the room where the magnetometer is measuring or just dropping a nail ;-)

During 2017-01-11 and 2017-01-29, the sensors were operated in an empty 40-liter cartboard box. The box was stored in a modestly temperatur stable storage. A DS18B20 temperature sensor (0.5oC accuracy, 0.0625oC readout) was placed inside the box. A 672-hr correlation with between the magnetic flux sensors and the temperature sensor:

On 2017-01-29, the cartboard box was filled with styrofoam pellets, with the temperature sensor immersed in the center of the box. A new temperatur measurement period has started.
From 2017-02-06 on, we experienced and interesting 15 deg Celsius temperature drop of the outside temperature. The SAM thermometer followed, registering a steady drop of about 2 deg Celsius. Since these measurements were accompanied by a rather quite magnetic period, they were used to determine a preliminary temperature dependency of the sensors. From 2017-02-07 on, this dependency is experimentally corrected for in the data.

After an upgrade to a better insulated EPS box, a new calibration run was performed on March 5/6:

A first, 68-day correlation vs planetary Kp:

Calibrating the local K scale: In his Geomagnetism Tutorial, Whitham Reeve describes a method suggested by Dirk Lummerzheim to develop a K-index scale for a particular station:

  1. Determine the Kp-index values for several 3-hour time periods over several days.
  2. Determine the field values (nT) at a nearby magnetometer station corresponding to the same time periods and days.
  3. For each Kp-index value, compare the range of corresponding field values to the range limits, or multiples of the range limits, described above (the multiple is the same for each Kp-index value and does not have to be an integer)
  4. Assign the appropriate range limits to each local K-index value so they resemble the Kp- index to the extent possible
When using local K-indices, also artificial disturbances will enter. Nevertheless, for illustration the plot below collects locally measured disturbances and shows their distibutions for individual Kp indices. The yellow bars represent the full dataset, the green bars a particularly undisturbed subset taken during 00 UTC and 06 UTC.